I get amazing chroma-keys with my SONY DSR-170 and my BlackMagic video=card. I record in 10b Uncompress the Video-out, what is a full broadcast composite video signal. The image I get is millions of miles away from the one I get recorded in the tape, and to make the chroma is really easy in AE or Shake. Of course, I need to have my CPU no far from the camera.
Anyway, if you have to make a chroma-key from DV footage, try to apply a Chroma-resampling filter, or blur (horizontaly only), the blue channel, or green if you are working with a green screen. just a bit.
Salud,
Rafael